The matter is to improve the audio rendering of the lowest frequencies in an equipment comprising an acoustic enclosure provided with an electro-dynamic loudspeaker for the bass, often called a woofer.
Such a loudspeaker produces a sound wave in response to an electrical signal applied as an input, via an electrical/mechanical conversion (by a coil through which an electrical current is circulated and which is plunged in a magnetic field created by polar parts and a magnet) and then a mechanical/acoustical conversion (by displacement of a membrane linked to the coil and that creates the sound wave).
Such loudspeakers are generally installed in open enclosures (ported system) or in closed enclosures, but are always more or less limited in the rendering of the lowest frequencies, the low limit (or cut-off frequency of the enclosure) depending on the loudspeaker size, the enclosure volume and the type of mounting used.
To artificially increase the rendering of the low frequencies and to cornpensate for the attenuation of the lowest frequencies near and below the enclosure cut-off frequency, various methods, often called “bass-boost”, have been proposed to artificially increase the rendering of the low frequencies, with accordingly a best rendering of the music for the listener.
The simplest technique consists in increasing the level of the electrical signal in the low frequencies by an appropriate, analog or digital, filtering. The bandwidth is then more extended in the low frequencies than without processing. Such solution has however limitations because the loudspeaker membrane excursion, i.e. the amplitude of its displacement with respect to its equilibrium position, becomes rapidly too high, with a risk of damaging the loudspeaker, and at the very least, a risk of introducing, for excessive excursion values, distortions that rapidly deteriorate the audio signal rendering quality. A second limitation relates to the output electrical power: indeed, the amplifiers that deliver the voltage to the loudspeaker terminals have intrinsic limits. If a bass-boost filter is used, in the low frequencies (where the energy is the highest), the signal will reach more rapidly the limits of the amplifier, with a risk of saturating the latter or outputting a clipped sound.
The EP 2 113 913 A1 (Parrot) proposes a technique based on a psychoacoustic property of the human ear, which makes it possible to perceive low frequencies even if they are not actually reproduced by the loudspeaker. Indeed, the perception of the height of a sound is not only linked to the presence of the fundamental frequency in the signal, by also to that of higher harmonics of this frequency. The technique proposed consists in reinforcing such harmonics, located above the acoustic enclosure cut-off frequency, so as to produce the illusion of a reinforcement of the fundamental frequency, that is not or not much reproduced. Such technique is well appropriate to equipments comprising small-diameter loudspeakers, such as the portable devices or the automotive vehicle equipments. But insofar as it introduces in the sound signal components that was not therein at the origin, it does not suit to the “high fidelity” equipments that, by principle, attempt to reproduce the sound signal in the truest manner possible and with the minimum of alterations.
The US 2008/0175397 A1 describes a system for extending downward the reproduction bandwidth of an acoustic enclosure, dynamically as a function of the power of the signal to be reproduced. The amplification chain comprises a dynamic high-pass filter whose cut-off frequency varies as a function of the power of the input signal (when the signal is stronger, the cut-off frequency is raised to reduce the bass, and vice versa). The sofiltered signal is then applied to a limiter stage whose gain varies dynamically as a function of four parameters calculated in parallel according to linear models, independently from each other: loudspeaker excursion, air speed in the port, detection of an audible saturation of the amplifier, and temperature of the loudspeaker coil. But the fact to use a limiter, which operates on the totality of the signal, has the drawback to introduce nonlinearities and thus to introduce the harmonic distortion. Furthermore, the parallel calculation, independently from each other, of the various factors conditioning the gain of the limiter cannot take into account the interaction between these parameters, nor the nonlinear behavior of a loudspeaker. The EP 1 799 013 A1 proposes a modeling of the loudspeaker behavior, and a compensation for the nonlinearities of the latter, in order to have a system that produces less audio distortions and that provides a better quality of listening by making the loudspeaker membrane displacement the more linear as possible. But it has not for object to dynamically extend the loudspeaker bandwidth in the area of the lowest frequencies, nor to compensate for the physical limitations of the response of the latter in this spectrum area near and below the acoustic cut-off frequency of the loudspecker/enclosure unit.
The US 2005/0207584 A1, U.S. Pat. No. 4,327,250 A and U.S. Pat. No. 6,201,873 B1 describe other techniques than filtering, equalizing or digital processing of the signal, aiming at predicting the loudspeaker response and at limiting if needed the level of the signal to be reproduced, so as to prevent from excessive excursions of the loudspeaker or from distortion-generating nonlinearities. But these other systems also suffer from all or part of the above-mentioned drawbacks.